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Audio amplifiers are used in equipment such as public address systems, sound recorders, sound reproducers and radio and television sets. The frequencies of the signals amplified are in the range of 10 to 20,000 Hz.
The input circuit of an amplifier may draw current from either the input device or the previous stage. In this respect, each amplifier is considered as either a current or a power amplifier operating at a current or a power level higher than the previous stage and lower than the following stage. Preamplifiers usually operate at power levels measured in microwatts. Driver stages usually operate at power levels measured in milliwatts. Power stages usually operate at power levels measured in hundreds of milliwatts or in watts. These power levels are only approximate; the equipment in which these stages are used determine the power levels of the preamplifier, the driver, and the power stage.